Published in: Proceedings of the 28th International Conference on Phenomena in Ionised Gases (ICPIG 2007) July 2007, Prague, Czech Republic

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1 Application of atmospheric pressure glow discharge (AGD) for deposition of thin silica like films on polymeric webs Starostin, S.A.; Aldea, E.; Vries, de, H.W.; Creatore, M.; van de Sanden, M.C.M. ublished in: roceedings of the 28th International Conference on henomena in Ionised Gases (ICIG 27) 15-2 July 27, rague, Czech Republic ublished: 1/1/27 Document Version ublisher s DF, also known as Version of Record (includes final page, issue and volume numbers) lease check the document version of this publication: A submitted manuscript is the author's version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. eople interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website. The final author version and the galley proof are versions of the publication after peer review. The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication Citation for published version (AA): Starostin, S. A., Aldea, E., Vries, de, H. W., Creatore, M., & Sanden, van de, M. C. M. (27). Application of atmospheric pressure glow discharge (AGD) for deposition of thin silica like films on polymeric webs. In F. J. Gordillo-Vazquez, R. M. ick,. Helfenstein, J. Schmidt, M. Simek, S. ekarek, & V. rukner (Eds.), roceedings of the 28th International Conference on henomena in Ionised Gases (ICIG 27) 15-2 July 27, rague, Czech Republic (pp /752). Bristol, United Kingdom: Institute of hysics. General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal? Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Download date: 18. Jan. 219

2 Eindhoven FUJIFILM, Application of atmospheric pressure glow discharge (AGD) for deposition of thin silica-like films on polymeric webs S.Starostin 1, E.Aldea 1,2, H. de Vries 2, M.Creatore 1, M.C.M. van de Sanden 1 1 University of Technology, Department of Applied hysics,.o. Box 513, 56 MB, The Netherlands 2 TRL,.O. Box 9156, 5 LJ Tilburg, The Netherlands Silica-like films were deposited on EN and ET polymeric foils in atmospheric pressure glow discharge (AGD) in a roll-to-roll reactor open to ambient air. AGD was ignited in a mixture of inexpensive carrying gas argon and nitrogen with oxygen and hexamethyldisiloxane (HMDSO) precursor. The uniform diffuse glow in various gas mixtures was sustained by utilizing electronic stabilization network. AGD operation in air for a present setup was demonstrated. The contribution of different deposition mechanisms to the resulting silica-like film is discussed. The dependence of film structure and chemical composition on the conditions during deposition process was studied by means of SEM, ATR-FTIR and XS analysis. The influence of oxidant concentration on the deposited film properties is analyzed and discussed. Figure 1. Schematic drawing of the experimental setup 1. Introduction Atmospheric discharges are nowadays in focus of serious research activity. This is mainly induced by the new promising applications possibilities for high pressure non-thermal plasmas. One of the traditional and successful fields of gas discharge application is plasma enhanced chemical vapour deposition of thin films (ECVD). Implementation of the atmospheric pressure discharges for ECVD can be advantageous, but also imposes scientific and technological challenges. Atmospheric pressure surface processing, including thin film deposition or surface modification, is primarily attractive because it allows a considerable cost reduction by avoiding usage of vacuum equipment and by offering a possibility of in-line roll-to-roll production. However, treatment of large area substrates requires creation of considerable volumes of uniform nonthermal plasma, which is not a trivial task at atmospheric pressure. One of the ways to produce non-thermal atmospheric plasma, allowing direct treatment of the large surface areas, is dielectric barrier discharge (DBD). Unfortunately, the most common form of high-pressure dielectric barrier discharge is a nonuniform filamentary mode, when discharge splits in many tiny current channels. The existence of uniform diffuse mode of DBD (suitable for high pressure ECVD) is usually strongly restricted by gas mixture composition, dissipated power and operation frequency. Often this rare uniform mode of atmospheric DBD is referred to atmospheric pressure glow discharge or AGD [1]. Despite apparent potential of the AGD for high pressure ECVD, till now there are not many reports about successful implementation of this promising technology for deposition of silica-like films. This is directly related to the difficulties of sustaining uniform mode of the discharge at atmospheric pressures in arbitrary gas mixture. Gas mixtures for deposition of oxide films usually should contain electronegative oxidant which is easily triggering transition to a filamentary mode. It is known that AGD can be obtained in pure gases for instance N 2 [2] and Ar or gas mixtures with high He content [3]. In the last case high costs of the gas mixture are making industrial application difficult. In a recent article [4] authors are utilizing uniform Townsend mode of the atmospheric DBD for deposition of silica-like coatings. The difference between Townsend DBD and AGD, studied here, is in that atmospheric glow discharge is characterized by higher current and specific power

3 3 3 4 Current [A] Voltage [V] Intensity [au] 2 1 N 2 Ar+N 2 + +HMDSO N 2,Ar 3.5x1-5 4.x x1-5 5.x1-5 Time [s] -3 Figure 2. Voltage-current waveforms for AGD in air densities, considerable space charge and formation of the electrode sheaths. In the present work we focus our studies on the AGD deposition of thin (~ 1 nm) Si -like films on polymers. One of the most known applications of these inorganic films is for gas/moisture diffusion barrier layers on polymers, but also other applications are very much appealing, e.g. scratch resistant coatings and optical coatings. Here it is our aim to investigate how the properties of the deposited films depend on the discharge conditions. 2. Experimental setup Schematic picture of the experimental setup is shown in Fig.1. The dielectric barrier discharge was ignited between two plane-parallel electrodes made from stainless steel and covered with dielectric barrier. The dimension of the discharge area was 4x15 mm 2 The electrode gap was 1 mm. A polymer foil (1 µm thick) treated in the discharge served the purpose as dielectric barrier. Both top and bottom electrodes were equipped with independent roll-to-roll foil transport systems. Two different polymer foils were tested as a substrate material in the present investigation: polyethylene terephthalate (ET) and polyethylene-2,6 naphthalate (EN). The DBD was powered by a RF LF-5 generator. Electrical circuit included matching elements together with discharge stabilization subcircuit. Discharge stabilization allows to sustain a uniform discharge avoiding transition to filamentary regime [5-7]. The operation frequency of the RF generator was tuned in the range of 1-2 khz to achieve better power coupling into discharge. A typical voltage amplitude between the electrodes was kv and the maximum value of NO Wavelength (nm) Figure.3 Optical emission spectrum of the AGD in Ar:N 2 : :HMDSO mixture discharge current peak was in the range of 4 8 A. The electrical characteristics of the discharge were monitored by a Tektronix TDS 334B oscilloscope equipped with Tektronix 615A voltage and Tektronix 616 current probes. The gas mixtures used in present invesigation contained argon (8%), nitrogen (2%), oxygen and hexamethyldisiloxane (HMDSO) as a precursor. Mixture of argon and nitrogen as carrier gas was used in this work to reduce the breakdown voltage and to allow operation of the system at 1 mm interelectrode gap with voltage amplitude of approx 2 kv. Operation of the AGD in pure nitrogen and air is also possible in the studied reactor if discharge gap is reduced. The total gas flow rate was 12 slm with a fixed precursor content of 5 ppm. The flow rate of the carrier gas and oxygen was controlled by mass flow meters (Bronkhorst). HMDSO precursor was injected using an evaporation unit (Bronkhorst). AGD discharge uniformity was monitored by a Dalsa EC-11-5H4 sensitive array camera with 13 µs frame integration time. Deposited coatings were analyzed by attenuated total reflection Fourier transform spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XS) and scanning electron microscope (SEM). Film thickness was controlled by spectroscopic ellipsometry (SE). 3. Results and discussion The implementation of the electronic stabilization network allows achieving a diffusive atmospheric plasma glow discharge (or AGD) in a large variety of gas mixtures, including ambient air. During the breakdown phase the plasma has a negative dynamic resistance and is very prone to the

4 Figure 4. SEM image of the AGD deposited silica-like film on EN substrate development of instabilities and the filamentation growth. However, the filament current can be damped and the current of the stable plasma variety enhanced via drop of the displacement current. In the studied reactor a fast falling displacement current was generated at the plasma breakdown and plasma cut-off using a LC parallel circuit with a saturated inductance [5-7]. The observation of the discharge with the sensitive array camera, was demonstrating uniform glow without visible filaments for all regimes of deposition under investigation. Another fingerprint of the AGD is the discharge current waveform, which was showing only one relatively broad current peak per half-period of the voltage between the electrodes. An example of the voltage-current waveforms for AGD in air (.5 mm discharge gap) can be seen in Fig.2. Considering deposition mechanisms in a roll-toroll reactor with moving polymer substrate and gas flow, one can distinguish three different pathways which are simultaneously contributing to the film formation: a) ionic deposition, when ionized products of decomposed precursor drift in electric field towards surface; b) diffusive deposition of neutral radicals produced in plasma and afterglow phases and c) deposition of large particles or dust. Each of these mechanisms leads to coatings with characteristic composition, structure and location in the discharge area (due to the gas flow). The fastest of the mentioned processes is ion drift. For the electric field, corresponding to experimental conditions, and atmospheric pressure ion drift time to the surface can be estimated as few µs. However because of the relatively low current density in AGD, this mechanism can not contribute significantly to the growth of the film bulk, although it may be responsible for the formation of interfacial layer between polymeric substrate and film. Characteristic time for diffusion of the radicals in our experimental conditions is in order of several ms. Radicals diffusion to the substrate can be considered as a main pathway for formation of dense films [4]. The rate of large particles growth in gas phase depends strongly on plasma chemistry and specific input power [4]. The deposition of particles is generally undesirable, because it leads to porous, powder-like coatings. As a result of above mentioned processes in a roll-to-roll system with a moving foil a film is formed which consists of mixed layers with different properties. Variation of the discharge conditions can change contribution of the mentioned processes and therefore affect characteristics of the resulting film. For deposition of silica-like coatings in high pressure discharge, the gas mixture typically contains the precursor, the oxidant and the carrier gas. Molecular oxygen can be used as an oxidant or O-containing molecules, N 2 O, can be used [4]. Atomic oxygen radicals and excited oxygen molecules formed in the discharge plasma are necessary to produce stoichiometric silica film. Moreover, atomic oxygen is promoting precursor decomposition and carbon elimination both in gas phase as well as from the growing film. Thus, the ratio of precursor to oxygen flow rates controls in many ways the plasma chemistry and the resulting properties of the deposited film. Typical optical emission spectrum of the AGD in the gas mixture of Ar:N 2 : /HMDSO is shown in Fig. 3. As one can see the spectrum is dominated by the molecular nitrogen bands and atomic Ar lines. Molecular bands attributed to NO can be seen in UV part. It can be noted that CO and C bands are not easily detectable. Comparing to the spectra of the low pressure HMDSO containing discharges [8], features attributed to the molecular or atomic hydrogen are not pronounced in AGD spectrum. SEM image of AGD deposited silica like film on EN substrate is shown in Fig. 4. It shows that smooth particle-free and pinhole-free coatings can be obtained in atmospheric discharge. By means of ATR-FTIR and XS analysis we observe that, in the absence of oxygen, HMDSOlike organic films (polymer-like) are deposited. Addition of oxygen leads to increase progressively in the silicon dioxide character of the film and to the decrease in carbon content in the deposited film. This can be seen in Figs. 5 and 6, which show ATR- FTIR spectra and XS data for the coatings deposited at different oxygen concentrations in the

5 Normalized absorbance Si-O-Si Si-CH 3 Si-(OH) Si-(CH 3 ) n - % -.25% - 5.% - 2.% Wavenumber (cm -1 ) Atomic concentration (%) C Si O in gas mixture (%) Figure 5. ATR-FTIR spectra of the films deposited at various oxygen content in the gas mixture gas mixture. The reduction in carbon content in the coating can be traced by observing absorption peaks corresponding to Si-CH 3 at 84 cm -1 and at cm -1 (corresponding to Si-(CH 3 ) n groups with n = 1,2,3) in ATR-FTIR spectra. It is known that the absorption peak assigned to Si-(CH 3 ) n groups shifts towards higher wavenumbers with decrease in n, which also implies an increase in - SiO x (CH 3 ) 3-x (cf. for instance [9]). In our experiments such shift is observed with an increase in oxygen percentage in the gas mixture. Both XS and ATIR-FTIR analysis shows that a limited increase in oxygen up to 2 % in gas mixture cannot totally eliminate carbon from the deposited film. Similar behavior of the residual carbon content in the film as a function of oxidant to precursor ratio (N 2 O/HMDSO) was observed in [4]. According to our experimental observations, the carbon content in the film can be decreased even further with increase in specific power deposition in AG discharge. 4. Conclusion Uniform large area silica-like coatings on the polymer substrate in roll-to-roll configuration were deposited at atmospheric pressure utilizing an AG barrier discharge configuration. The atmospheric discharge deposition system works at relatively low flow rate of the Ar/N 2 carrier gas and it is open to ambient air. This makes the system inexpensive for industrial applications. By changing HMDSO/oxygen ratio in the gas mixture it is possible to tune the composition of the deposited film from organic polymerized HMDSOlike to inorganic silica-like coatings with negligible residual carbon content. Figure 6. Composition of the AGD deposited film as a function of oxygen content in the gas mixture. Data of XS analysis. 5. References [1] S. Okazaki, M. Kogoma, M. Uehara, Y. Kimura, J. hys. D: Appl. hys. 26 (1993) 889. [2] D.Trunec, Z.Navrátil,.Stahel, L.Zajíčková, V.Buršíková, J.Čech, J. hys. D: Appl. hys., 37 (24) [3] R. Foest, F.Adler, F.Sigeneger, M.Schmidt, Surface and Coatings Technology, (23) 323. [4] F.Massines, N.Gherardi, A.Fornelli, S.Martin, Surface and Coatings Technology, 2 (25) [5] H.de Vries, F.Mori, E.Aldea, M.C.M van de Sanden US , E (24), Fuji hoto Film B.V. [6] J.Bouwstra, E. Aldea, M. C. M. van de Sanden, H de Vries E A1 (25), Fuji hoto Film B.V. [7] H.de Vries, J.Bouwstra, E. Aldea, M. C. M. van de Sanden,.eeters E A1 (26), Fuji hoto Film B.V. [8] A.Granier, M.Vervloet, K.Aumaille, C.Vallee, lasma Sources Sci.Technol. 12 (23) 89 [9] X.D.Zhu, F.Arefi-Khonsari, C.etit-Etienne, M.Tatoulian, lasma rocess. olym. 2 (25) 47.